1. Field
Embodiments described herein relate generally to a nonvolatile semiconductor memory device.
2. Description of the Related Art
Current NAND type flash memories include a floating gate electrode in each memory cell and execute writing or erasing by charging or discharging electrons into or from the floating gate electrode. It is possible to set a plurality of threshold voltage states (data states) by controlling the amount of electrons to be charged into the floating gate electrode. In recent years, NAND type flash memories for storing 2 bits (4 values) or 3 bits (8 values) in one memory cell have been developed and mass-produced.
In a NAND type flash memory, the threshold voltage of a memory cell changes depending on the amount of electrons charged into the floating gate electrode. In a reading operation, a certain voltage (an intermediate voltage between a plurality of threshold voltage distributions) is applied to a selected word line, while a reading pass voltage Vread (a voltage that makes the memory cells conductive regardless of the data stored in the memory cells) is applied to non-selected word lines. The reading operation is executed by determining whether or not a current flows through a NAND cell unit in response to the application of these voltages.
In order to let flow a sufficient current through non-selected memory cells, the reading pass voltage Vread is set even higher than the upper limit value of the highest threshold voltage distribution by approximately 2 V. In recent years in which miniaturization has advanced, it has been occurring that the potential of the floating gate electrode of a selected memory cell rises due to influence of the reading pass voltage Vread applied to adjoining non-selected word lines. Hence, there occurs a phenomenon that the threshold voltage of the selected memory cell seems as if it has become lower. As a result, the selected memory cell which should not become conductive in response to the threshold voltage of the data retained therein becomes conductive, posing a risk that erroneous data might be read out.
Moreover, with the advancement of miniaturization, there may also occur a phenomenon that a threshold voltage distribution seems to be wider due to influence of charges that have been left around the floating gate electrode of a selected memory cell since a writing or erasing operation. When the threshold voltage distribution becomes wider at its upper limit side, the selected memory cell which should become conductive in response to the threshold voltage of the data retained therein does not become conductive, also posing a risk that erroneous data might be read out.